Magnetic encoders are widely used to detect the rotation speed, angle and position in many precise control systems. They are quite rigid with simple structures, offering reliable operation in adverse environments where high vibration, temperature, moisture or dust may exist. Since the motor size is getting smaller, a strict condition is required for magnetic encoders with high resolution. Therefore, conventional magnetic encoders with wide magnetic pole pitch are not enough for using in precise control systems anymore. A precise magnetic encoder consists of a magnetic reading device and a multi-pole magnetic component with fine magnetic pole pitch. Its resolution is determined by the size of the magnetic pole pitch. The dimension of a monopole means the magnetic pole pitch. A smaller of the magnetic pole pitch provides a higher resolution in detection.
The signals in the multi-pole magnetic component can be detected using a magnetic reading device like Hall element or MR (magneto-resistance) element. The position and speed of a moving object can be obtained from detecting the rotation angle and direction of magnetic encoder. Generally, the multi-pole magnetic component with fine magnetic pole pitch in the magnetic encoder is achieved by magnetization. An unmagnetized magnetic component is placed onto the surface of a magnetizing head. The magnetizing coil is wound on the magnetizing head and the winding pattern is depended upon the design of the magnetizing head. Connecting the terminals of magnetizing coil to a magnetization machine which can provide the magnetizing current. After releasing a magnetizing current, the strong magnetic field is induced to magnetize the magnetic component. Therefore, a precise magnetizing head and a magnetization machine are required for narrowing the magnetic pole pitch of the multi-pole magnetic component. Traditionally, the multi-pole magnetizing head is obtained from the line-cutting process and the smallest magnetic pole pitch can be acquired about 1 mm by magnetization using the magnetizing head. The magnetic pole pitch of less than 1 mm is very difficult to achieve because it is limited by the precision of the machining tools and the bending angle of the magnetizing coils.
As described in the U.S. Pat. No. 4,920,326, the surface of the magnetizing head is divided into eight equal parts (16, 16′, 18, 18′, 20, 20′ . . . 30 and 30′) by line-cutting process and the magnetizing coil (34) is wound into the groove as shown in FIG. 1. The grooves are located between any two parts. An alternate multi-pole magnetic field distribution is formed with an appropriate arrangement of the magnetizing coil. Both terminals (36 and 38) of the magnetizing coil are connected to a magnetization machine which can provide the magnetizing current. A strong magnetic field is induced instantaneously after the magnetization machine releases a magnetizing current. Then the magnetic component (40) is magnetized with a multi-pole structure. It is seen that the distance between the magnetic poles is limited by the machining technique and the minimum is about 1 mm. Besides, the insulating layer of the magnetizing coil can not withstand the stress and then breaks. It is caused by the large bending angle of the magnetizing coil being used in a magnetizing head with fine magnetic pole pitch. Therefore, a short circuit is happened on the bases (12 and 12′) of the magnetizing head. Since the bases are made of a ferromagnetic material with high permeability, the magnetizing coil and head are exploded frequently during the magnetization. Thus this way is very dangerous.
To overcome the limitation of 1 mm in the magnetic pole pitch, a new magnetization method is introduced using the single-pulse magnetizing technique like the magnetic recording technology. The disclosed in the proceeding of Electrical Electronics Insulation Conference and Electrical Manufacturing & Coil Winding Conference (Chicago '93 EEIC/ICWA Exposition, P.237-242, 1993) is shown in FIG. 2. The magnetizing coils (200) are wound on the magnetizing head (201). The leakage of the magnetic field from the magnetizing head is used to write the magnetic pole pairs (i.e. N and S pole) onto the surface of the magnetic component (202). The magnetic pole pitch of less than 1 mm is accomplished successfully. Before magnetization, the magnetic component is mounted on a base which is usually supported and rotated by a high precision spindle motor. Then, the magnetic component is magnetized with magnetic pole pairs intermittently controlling by using a magnetization machine. The precise position control of the spindle motor is highly required; otherwise, an asymmetric magnetic file distribution will happen in the multi-pole magnetic component after magnetization and it is not good for subsequent processing of signals. In addition, the dimension of the magnetic component must be controlled uniformly. During the magnetization, the magnetic component will often collide with the magnetizing head if its radial run-out is too large and thus result in damages both of them. Moreover, the leaking gap in the magnetizing head and the air gap between the magnetizing head and the magnetic component have to be properly controlled. These are the key factors to affect the dimension of the magnetic pole pitch in magnetization. As the magnetic pole pitch on the magnetic component gets smaller, the leaking gap in the magnetizing head has to be narrowed as well. The air gap between the magnetizing head and the magnetic component must be appropriately tuned to obtain the desired magnetic pole pitch. Therefore, the manufacturing of a multi-pole magnetic component can be accomplished only under a precise controlling in magnetization and it is difficulty.
Besides, the waveform of the magnetizing current from the magnetization machine is required to modify in order to magnetize the magnetic component with different magnetic properties using the single-pulse magnetizing technique. Because the waveform of the magnetizing current highly depends upon the magnetic material property and this can be achieved only through a precise magnetization machine. In addition to controlling the tiny radial run-out and the material homogeneity on the magnetic component, it has to be mounted on a spindle motor under a precision position control. The desired magnetic pole pitch can be accomplished by tuning an appropriate leaking gap and the magnetizing air gap during the magnetization. Despite the fact that this technique can narrow the magnetic pole pitch to around 200 μm, the process is very complicated and difficult. The high precision machining, the techniques for making the precise magnetizing head and the magnetization machine are essential, and therefore the single-pulse magnetizing technique is costly and not economical at all.